Generating a tunable narrow electron beam comb via laser-driven plasma grating

The interaction of multiple laser beams with plasmas has revealed various nonlinear effects, exhibiting novel features and finding numerous applications in different interaction configurations. For instance, four-wave mixing can result in phase-conjugating reflection,¹ and the use of two laser pulses can accelerate energetic particles.^2–7 Attosecond x-ray pulses can also be generated,⁸ and terahertz radiation can be produced using a two-color laser scheme.^9,10 These interactions can occur when the laser beams are either parallel or crossed at specific angles. Moreover, recent studies have demonstrated that the intersection of two nonrelativistic laser beams with a plasma can induce a stable grating-like electron density modulation called a plasma Bragg grating (PBG).^11–14 This grating has diverse applications, including the manipulation of ultrashort intense laser pulses,^15–17 generation of electron bunches,¹⁸ production of high-density electrons,¹⁹ and triggering of electron injection in the laser wakefield.²⁰ However, when the intensity of the pump lasers reaches relativistic levels, new nonlinear phenomena arise during the interaction process.

In this work, we propose a novel method for generating a high-density narrow electron beam comb (EBC) in the presence of a plasma grating. Our approach involves the interaction of two cross-propagating laser pulses at specific angles with critical-density plasmas. When these intense laser pulses intersect in the plasma, they create an interference field in the region of overlap, leading to the formation of an electron plasma grating. This grating introduces a charge imbalance between electrons and ions, resulting in strong mutual repulsion among the electrons. This repulsion causes Coulomb expansion of the electrons, with some of them expanding into the vacuum. Under the influence of the transverse standing wave field, a periodic accelerated EBC is generated. Our research demonstrates that the interference field can effectively guide and collimate these electron beams, allowing them to stably propagate for tens of femtoseconds. Furthermore, the width of the individual electron beams along the y direction can be reduced to the nanoscale range, with energy levels reaching several MeV and density levels reaching the critical density of the initial plasma. Additionally, the characteristics of the EBC can be tailored by adjusting the initial laser parameters.

Figure 1 illustrates the proposed EBC scheme based on a plasma grating. The scheme is simulated using a 2D3V particle-in-cell (PIC) method with the LAPINE code.²¹ The simulation involves two circularly polarized (CP) laser pulses, with the same strength parameter a_L = 2, wavelength λ = 800 nm, spot radius b = 10λ₀, and pulse duration τ = 30T₀, where λ₀ = 1 µm is the unit of the simulation and T₀ is the laser period normalized with respect to λ₀. At t = 0, the two CP laser pulses intersect on the left side (z = 0) of the plasma target at an angle θ = 10°. The peak intensity of the laser is 8.56 × 10¹⁸ W/cm². The plasma target consists of a uniform subcritical density plasma layer with density n₀ = 0.1n_c and thickness 15λ₀, where $nc=1.1×1021/λ02cm−3$ is the critical density. The plasma layer is located in 8 < z/λ₀ < 23. The simulation box is 40λ₀ × 25λ₀, containing a 8000 × 5000 grid, with 16 ions and 16 electrons per cell. The ion–electron mass ratio is 1836.

To simulate the generation of the EBC, we first consider the propagation of laser pulses through a subcritical density plasma. Figure 2 illustrates the evolution of the EBC, showing snapshots of the normalized laser light field a_L(y, z, t) at time t = 14T₀ [Fig. 2(a)] and the electron density at different times t = 14T₀, 18T₀, and 22T₀ [Fig. 2(b)–2(d)]. As the two laser pulses propagate forward and intersect on the plasma target at z = 8λ₀, their y components overlap and induce an interference field. We observe that the ponderomotive force of the interference field starts to expel electrons from the high-intensity regions, causing them to accumulate in the wave nodes and form a narrow density peak with a density several times higher than the background plasma. Subsequently, a spatially periodic modulation in electron density, known as an electron plasma grating,¹¹ is driven by the ponderomotive force, as depicted in Fig. 2(b). During this period, the massive ions remain stationary while the local non-neutrality of charge is established. The unneutralized electrons within the narrow high-density electron peak interact through mutual repulsion, triggering the Coulomb expansion process and inducing their acceleration. Some electrons move transversely and neutralize with the ions, while some move forward, sustaining the persistent Coulomb expansion. Additionally, a portion of backward-moving electrons overflows the target surface, forming an EBC.

At a later time t = 18T₀, under the continuous influence of the ponderomotive force of the interference field, most electrons between the peaks of the electron plasma grating are expelled from the high-intensity regions, resulting in the formation of several electron cavities, as observed in Fig. 2(c). Notably, the Coulomb expansion process of electrons persists from the density peak. We can observe the electron beams continuously emerging from the target surface with a speed of ∼0.6c, while the electron density ripple structures induced by the transverse neutralization effect of electrons and ions begin to manifest in the plasma layer. The single electron beam of the EBC is compressed to an extremely narrow width in the nanoscale range under the guidance of the standing field in the y direction. The EBC can stably propagate longitudinally in vacuum for tens of femtoseconds. Eventually, the mutual repulsion among the electrons leads to dissipation of the electron bunches, as shown in Fig. 2(d).

We now further explore the detailed process of generation of the EBC. In Fig. 3, we observe the normalized laser light field a_L(y, z, t) and electron density along the y direction at (a) z = 9.5λ₀ and (b) z = 7.74 λ₀ at time t = 18T₀. Figure 3(a) illustrates the periodic standing field and the density distribution of the electron plasma grating. The ponderomotive force of the standing field expels the electrons located in the wave antinodes from the high-intensity regions. As a result, these electrons accumulate in the wave nodes, giving rise to the formation of a plasma grating. The period of this grating, as seen in Fig. 3(a), is ∼2.3 µm, and the density of the electron peak is several times higher than the initial plasma density. Figure 3(b) shows the periodic standing field and density distribution of the EBC at z = 7.74 λ₀ in vacuum. The period of the standing wave remains the same as in the plasma layer. Notably, the width of an individual electron beam in the y direction is compressed to about 52 nm (the electron beam width mentioned later refers to the y direction). The densities of these electron beams can approach the critical density.

This simplified expression describes the intensity distribution of the standing wave field formed by the crossing propagation of two lasers with the same frequency and amplitude. The intensity I exhibits a periodic modulation along the y axis with wave vector q = 2k sin θ = 2π/Λ. Consequently, the ponderomotive force will induce a periodic modulation of the electron density with a spatial period Λ = λ/(2 sin θ). It is evident that the period of the plasma grating is determined solely by the wavelength of the laser and the angle of incidence. Specifically, for θ = 10° and λ = 800 nm, we can deduce that the period of the plasma grating is ∼2.3 µm, which is highly consistent with the simulation results obtained in Sec. II.

To demonstrate the influence of laser wavelength and incident angle on the formation of the EBC, we conducted additional simulations for comparison, as shown in Fig. 4. In Fig. 4(a), we used a laser wavelength of λ = 260 nm while keeping the other laser and plasma parameters the same as in the previous simulations. It is observed that the periods of both the electron grating and the electron beams are shorter than those in Fig. 3. The density of the individual electron beams decreases to ∼0.5n_c, and their width is compressed to about 10 nm. However, owing to the limitations of current laser technology, it is challenging to implement this experiment.

In Fig. 4(c), we changed the angle of incidence to θ = 20° while keeping the other parameters constant. We observe that the periods of both the electron grating and the electron beams are again shorter than those in Fig. 3. This result further confirms that the laser wavelength and angle of incidence are crucial factors that influence the periods of the plasma grating and electron beams. From the formula in Eq. (5) for the intensity of the standing wave, it is evident that this intensity exhibits a periodic relationship with respect to the incident angle, rather than a linear one. Consequently, the density and width of the generated electron beams also display oscillatory behavior with respect to the angle of incidence. For instance, when we varied the angle of incidence from θ = 5° to θ = 50° while keeping other parameters constant, the simulation results were in agreement with the predictions from Eq. (5). Considering that the period of the plasma grating is Λ = λ/(2 sin θ), it is evident that the angle of incidence should not be too small. If the angle is approximately zero, then the two laser beams will no longer interfere, leading to the absence of electron beam generation.

To investigate the influence of laser intensity on the EBC, Fig. 5 compares the peak density of the electron beams for three different laser field strengths. The results indicate that the period of the electron beams is not affected by the laser intensity. This supports the previous finding that only the laser wavelength and incident angle play a role in determining the period of the electron beams. It is observed that the peak density of the electron beams increases with increasing laser field strength, reaching 0.86n_c for a_L = 5 and 0.04n_c for a_L = 1. This trend can be attributed to the stronger ponderomotive force arising with higher-intensity lasers, which determines the density of the electron grating. Consequently, continuous Coulomb expansion is more likely to occur. Therefore, the generation of an EBC requires a sufficiently intense laser to expel electrons and create electron cavities that facilitate Coulomb expansion. The simulations also reveal that higher laser intensities result in a narrower compression of the electron beam, indicating a smaller scale in the y direction. Additional simulations were conducted to explore the impact of different laser durations on the generation of the EBC. The results are similar to those shown in Fig. 3.

To further investigate the mechanism of generation of the EBC, we tracked the trajectories of several groups of electrons that were induced by circularly polarized (CP) and linearly polarized (LP) laser pulses with the same laser intensity, as shown in Fig. 6. Our results demonstrate that both CP and LP laser pulses are capable of generating the EBC phenomenon. As already mentioned, during the formation of the plasma grating, the ponderomotive force initially pushes the electrons to form high-density peaks. The local charge non-neutrality between electrons and ions causes the electrons to experience a Coulomb expansion. Subsequently, some of the electrons expand into the vacuum region. We can observe from the electron trajectories in Fig. 6 that during the initial stage, under the influence of the laser polarization, the electrons move forward in a spiral pattern for CP laser pulses and in a zigzag pattern for LP laser pulses. Once the electrons expand into the vacuum, they become trapped by the stationary wave that forms along the y direction when the two laser pulses intersect on the plasma target. As a result, the EBC can be collimated and guided by the stationary wave for tens of femtoseconds. Furthermore, we compared the peak density of the electron bunches between CP and LP lasers and obtained similar results, as shown in Fig. 6(c). This suggests that laser polarization does not significantly affect the peak density of the electron bunches.

As already mentioned, the Coulomb expansion of the electrons causes the acceleration process. Owing to the continuous action of the ponderomotive force, most electrons accumulate in the electron peaks, resulting in continuation of the acceleration process caused by Coulomb expansion. The energy spectrum of the EBC, as shown in Fig. 7, demonstrates that the electrons are accelerated to energies ranging from ∼5 to 20 MeV at time t ∼ 40T₀.

In summary, the interaction of two circularly or linearly polarized laser pulses with a uniform under-critical-density plasma can generate a standing wave, leading to the formation of a spatially periodic plasma grating. This grating causes a local charge imbalance, resulting in a significant mutual repulsion among electrons and inducing Coulomb expansion. The guided and focused expansion of electrons into the vacuum leads to the creation of a high-density nanoscale narrow electron beam comb (EBC). The period of the plasma grating and EBC depends on the laser wavelength and incident angle, while the peak density is influenced by various laser parameters. It has been observed that the generation of high-density EBCs requires a broad range of specific laser and plasma parameters, suggesting the potential for stable EBC generation. Remarkably, these EBCs can propagate stably in vacuum for several tens of femtoseconds. They can serve as a valuable source of high-brightness short-pulse coherent radiation, including x-rays and gamma rays, enabling the exploration of ultrafast processes at the molecular level. Furthermore, the expansion process of electrons in the standing wave can generate x-ray and terahertz radiation, and the tunable standing wave can be used as the wiggler for a short-wavelength free-electron laser. However, further research is needed to gain a detailed understanding of the expansion process and higher-dimensional effects associated with this mechanism. In conclusion, the mechanism studied here exhibits promising potential for various applications and warrants further investigation.

This work was supported by the National Natural Science Foundation of China (Grant Nos. 12174410, 11991072, 11991074, 12225411, and 12105353), the CAS Project for Young Scientists in Basic Research (Grant No. YSBR060), the State Key Laboratory Program of the Chinese Ministry of Science and Technology, and the CAS Youth Innovation Promotion Association (Grant Nos. Y201952 and 2022242).

The authors have no conflicts to disclose.

Hetian Yang: Data curation (equal); Formal analysis (equal); Investigation (equal). Jingwei Wang: Conceptualization (equal). Shixia Luan: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Investigation (lead); Supervision (equal); Writing – original draft (lead). Ke Feng: Conceptualization (equal). Wentao Wang: Formal analysis (equal); Supervision (equal); Writing – review & editing (equal). Ruxin Li: Supervision (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.